CDs and DVDs (digital versatile disks) currently in widespread use each have a capacity of several hundreds of megabytes to several gigabytes. The development and commercialization of next generation DVDs (also referred to as high-definition DVDs) which are optical recording media having a larger capacity of several tens of gigabytes are under way in response to a recent increase of the quality of images. Along with a demand for smaller size and lower cost, an optical pickup which records data on and/or plays back data from a next generation DVD is desired to adopt a compatible technology for enabling itself also to record data on and/or play back data from DVDs and CDs which are conventional optical recording media using a common objective lens. However, the optical recording media differ from one another in used wavelength, numerical aperture (NA), and the thickness of a signal plane protecting substrate, as shown in Table 1. Accordingly, if a common objective lens is used, spherical aberration cannot be corrected, and it is impossible to normally record or play back information.
[Table 1]
TABLE 1Thickness ofOpticalUsedprotectingrecordingwavelengthsubstratemedium type(nm)NA(mm)Next4050.850.1generationDVD 1Next4050.650.6generationDVD 2DVD6500.6 0.6CD7800.451.2
FIG. 1 shows an optical pickup which records information on and/or plays back information from a plurality of optical recording media having specifications different from one another using a phase correcting element in order to solve the above-described problem (see Japanese Patent Application Laid-Open No. 2004-246931). In FIG. 1, an objective lens 1 is designed such that the wave front aberration to light having a wavelength corresponding to a next generation DVD is minimum on the signal plane of an optical recording medium (next generation DVD) 2-1 when the light is incident as a parallel light beam. A DVD hologram module 3 and a CD hologram module 4, in each of which light receiving and emitting units are packaged, are used as DVD and CD light sources. A light beam radiated from a next generation DVD semiconductor laser 5 is transformed into a parallel light beam by a collimator lens 6, passes through a deflection beam splitter 7 and DVD and CD dichroic prisms 8 and 9 for light beam separation, and changes its direction 90° by a prism 10. A wavelength plate 11 changes the state of polarization of the light beam from linear polarization into circular polarization, and an aperture restriction element 12 restricts the diameter of the light beam to a light beam diameter corresponding to the NA of a next generation DVD. The light beam passes through a phase correcting element 13 without changing its parallel light beam state (with an equiphase wave surface kept planar), comes incident on the objective lens 1, and is focused on the signal plane of the optical recording medium (next generation DVD) 2-1, thereby recording and/or playing back information. A light beam reflected from the optical recording medium 2-1 is transformed into a circularly polarized light beam having an opposite rotational direction and is transformed into a parallel light beam by the objective lens 1. The wavelength plate 11 changes the state of polarization of the light beam from circular polarization into linear polarization such that the resultant linearly polarized light beam is orthogonal to the original linearly polarized light beam. The light beam changes its direction 90° by the prism 10, is reflected by the deflection beam splitter 7, and is focused on a light receiving element 15 by a detecting lens 14. A light beam radiated from the DVD hologram module 3 is transformed into a parallel light beam by a coupling lens 16, is reflected by the dichroic prism 8, passes through the dichroic prism 9, and changes its direction 90° by the prism 10. The wavelength plate 11 changes the state of polarization of the light beam from linear polarization into circular polarization, the aperture restriction element 12 restricts the diameter of the light beam to a light beam diameter corresponding to the NA of a DVD, and the phase correcting element 13 corrects the spherical aberration to the light beam. The light beam comes incident on the objective lens 1 and is focused on the signal plane of an optical recording medium (DVD) 2-2, thereby recording and/or playing back information. A light beam reflected from the optical recording medium 2-2, which is a DVD, is transformed into a circularly polarized light beam having an opposite rotational direction and is transformed into a parallel light beam by the objective lens 1 and phase correcting element 13. The wavelength plate 11 changes the state of polarization of the light beam from circular polarization into linear polarization such that the resultant linearly polarized light beam is orthogonal to the original linearly polarized light beam. The light beam changes its direction 90° by the prism 10, is reflected by the dichroic prism 8, and is focused on a light receiving element of the DVD hologram module 3 by the coupling lens 16. A light beam radiated from the CD hologram module 4 is transformed into a predetermined divergent light beam by a coupling lens 17, is reflected by the dichroic prism 9, and changes its direction 90° by the prism 10. The wavelength plate 11 changes the state of polarization from linear polarization into circular polarization, and the aperture restriction element 12 restricts the diameter of the light beam to a light beam diameter corresponding to the NA of a CD. The light beam passes through the phase correcting element 13 without changing the shape of an equiphase wave surface, comes incident on the objective lens 1, and is focused on the signal plane of an optical recording medium (CD) 2-3, thereby recording and/or playing back information. A light beam reflected from the optical recording medium 2-3 is transformed into a circularly polarized light beam having an opposite rotational direction and is transformed into a predetermined convergent light beam by the objective lens 1. The wavelength plate 11 changes the state of polarization of the light beam from circular polarization into linear polarization such that the resultant linearly polarized light beam is orthogonal to the original linearly polarized light beam. The light beam changes its direction 90° by the prism 10, is reflected by the dichroic prism 9, and is focused on a light receiving element of the CD hologram module 4 by the coupling lens 17.
In the conventional example in FIG. 1, it is necessary for a light beam having at least one wavelength to configure the optical system using a finite optical system, and a light beam having the wavelength suffers from the problem of exacerbated spherical aberration during tracking of the objective lens 1. To set the angle of incidence from a finite optical system on the objective lens 1 to a desired value, intervals at which units of the optical system are arranged need to be set to a specific value. The process of giving phase shifts which are integer multiples of 2π to light beams having wavelengths corresponding to a next generation DVD and CD which are not exactly integer multiples by the common phase correcting element and allowing the light beams to pass through the common phase correcting element without changing the shape of an equiphase wave surface limits the types of glasses available for use in the phase correcting element to specific ones. Accordingly, the flexibility in configuring the optical system becomes extremely lower, and designing of the optical system becomes difficult.